It is desirable that a transportable folding bridge should have in its unfolded state, both a flush bottom face to permit passage of the bridge across rollers during launching and a flush top face for ease of traffic flow across the bridge.
Simple hinges, commonly applied to existing bridges and many other folding structures have a single hinge pin and bush which are necessarily axially located in the plane of one of the faces in order to provide a full 180.degree. rotation, and consequently the pin and bush must partially protrude from that face when the hinge is opened.
A more complicated type of hinge providing a flush surface at both faces when opened through 180.degree. is known for use with lightweight folding structures, e.g. table leaves or countertop flaps, in which the two adjoining portions of the structure are separately pinned to an intermediate swinging link in a plane lying between the two faces. This type of hinge opens erratically, rotation occurring indiscriminately at either one of the two hinge pins at any moment, and when a pair of the hinges are used in line, relative twisting of the two attached portions occurs. Consequently the hinge is only suitable for use with fairly lightweight structures that can be manually controlled during opening and closing.